Using an EQ is like doing a puzzle. It's there to solve problems.
Out of any processing tool/plugin you wish to use, the humble EQ is the one you must learn to master. It is the most valuable asset to any music producer's arsenal.
Do you know it is completely possible to only use EQ on a track and nothing else, yet still come out with a great sounding track?
EQ can do many things. It can help tidy up a muddy-sounding mix by filtering out the low end...
...It can also be used to boost frequencies to make them become more present in the mix...
...You can also use it to fix frequencies that are causing problems in your mix and eliminate any issues...
So, as you can see, it is absolutely imperative that you master the EQ and ensure that you completely understand it.
To elaborate on this further, I highly recommend you check out this great article from the guys over at Dubspot.
This article talks you through all the various EQ types as well as when and where they are best applied.
We also have a downloadable EQ cheat sheet available, which gives you a guide on what certain frequency areas mean.